Appendix II. 817 a finishing tool, when cutting screws in the lathe. It is, how- ever, sometimes used both to start and finish the thread, being held by the hand throughout the operations. It is easily seen that such a method has precisely the same objections as stock and dies, but even to a worse extent, for c drunken; threads, of varying pitch, are often produced by inexperienced workmen. This is because the tool has very short directing surface, even though that surface may have the correct angle. (5.) By Screwing Machine.—This method is the same in prin- ciple as stock and dies, and is merely a quicker and more automatic way of doing the same work. In Fig. 786 the general form of the machine is that of a short lathe with fixed headstock A, a clutch B for putting the mandrel in and out of gear quickly, and a powerful driving gear. The work is held in a concentric chuck c, and the screwing dies are in themselves a sort of concentric chuck, operated by the lever E. The mandrel being revolved, the cutting head D is brought over the work by the pinion and hand- wheel F until the dies are in position to start: the lever E is then pulled leftward by the advancing cutters, being partly helped by the workman's hand at E. -The operation may be repeated until the stud is of correct diameter, as indicated by the angle of the lever E. With plenty of lubrication the thread may be cut during one traverse, and when the dies are blunted they can be recut upon a master tap placed in chuck c. v Cutting Long-pitched Screws. — When the thread angle becomes 45° or over, it may be questionable policy to cut it in a lathe. Remembering that a screw is formed by an axial traverse and a rotation, either motion may be called the cut, the other being the feed; but the latter should always be preferably the slower or smaller motion. Thus, in the machine for rifling guns, the traverse being large and the rotation small, the tool is propelled axially, the holder being a long bar with the tool secreted in the end, in the manner known as the 'tiger's claw/ from the fact that it is sheathed on entering, and automatically shot out on the outward or cutting stroke. During withdrawal the tool bar is rotated by a rack and pinion, the amount of rotation being fixed by the inclination of a bar along which the rack arm travels. In such manner any long-pitched screw may be cut.